GM Crop Database

Database Product Description

676, 678, 680

Host Organism: Zea mays (Maize)

Trait: Glufosinate ammonium herbicide tolerance and fertility restored.

Trait Introduction: Microparticle bombardment of plant cells or tissue

Proposed Use: Production for human consumption and livestock feed.

Product Developer: Pioneer Hi-Bred International Inc.

Summary of Regulatory Approvals

Country	Food	Feed	Environment	Notes
United States	1998	1998	1998

Introduction Expand

Maize lines 676, 678, and 680 were developed to provide a reliable hybrid system based on nuclear male sterility. The transgenic lines were produced by genetically engineering plants to be male sterile and tolerant to the herbicide glufosinate ammonium, where herbicide tolerance was used as a selectable marker to identify the male sterile plants. These transformed maize lines contain an introduced gene (dam) from Escherichia coli that encodes the enzyme DNA adenine methylase (DAM). Expression of this gene in specific plant tissues results in the inability of the transformed plants to produce anthers or pollen, resulting in a male sterile plant.

The maize lines 676, 678, and 680 also contained the pat gene, isolated from the bacterium Streptomyces viridochromogenes, for use as a selectable marker. The pat gene encodes a phosphinothricin acetyl transferase (PAT) enzyme, which, when introduced into a plant cell, confers tolerance to the herbicide glufosinate ammonium. The pat gene was used as a selectable marker to identify transformed plants during tissue culture regeneration, and as a field selection method to identify the male sterile lines prior to flowering. Under field conditions, plants that were not male-sterile could be eliminated by application of the herbicide glufosinate ammonium. The novel hybrid system provided an efficient and effective way to identify male-sterile plants for use in hybrid seed production.

Glufosinate ammonium is the active ingredient in phosphinothricin herbicides (Basta®, Rely®, Finale®, and Liberty®). Glufosinate chemically resembles the amino acid glutamate and acts to inhibit an enzyme, called glutamine synthetase, which is involved in the synthesis of glutamine. Essentially, glufosinate acts enough like glutamate, the molecule used by glutamine synthetase to make glutamine, that it blocks the enzyme's usual activity. Glutamine synthetase is also involved in ammonia detoxification. The action of glufosinate results in reduced glutamine levels and a corresponding increase in concentrations of ammonia in plant tissues, leading to cell membrane disruption and cessation of photosynthesis resulting in plant withering and death. The PAT enzyme detoxifies phosphinothricin by acetylation into an inactive compound.

Summary of Introduced Genetic Elements Expand

Code	Name	Type	Promoter, other	Terminator	Copies	Form
dam	DNA adenine methylase	MS	Maize 512del anther-specific promoter	Solanum tuberosum proteinase inhibitor II (pinII) transcription termination signal	1 (676); 3 (678); 1+3 partial (680)	Native
pat	phosphinothricin N-acetyltransferase	HT	CaMV 35S		2 (676); 1+1 partial (678); 1 (680)	Native

Characteristics of Zea mays (Maize) Expand

Center of Origin	Reproduction	Toxins	Allergenicity
Mesoamerican region, now Mexico and Central America	Cross-pollination via wind-borne pollen is limited, pollen viability is about 30 minutes. Hybridization reported with teosinte species and rarely with members of the genus Tripsacum.	No endogenous toxins or significant levels of antinutritional factors.	Although some reported cases of maize allergy, protein(s) responsible have not been identified.

Donor Organism Characteristics Expand

Latin Name	Gene	Pathogenicity
Streptomyces viridochromogenes	pat	S. viridochromogenes is ubiquitous in the soil. It exhibits very slight antimicrobial activity, is inhibited by streptomycin, and there have been no reports of adverse affects on humans, animals, or plants.

Modification Method Expand

These transgenic maize lines were produced using a microparticle bombardment (biolistic) technique to introduce the genes of interest, followed by plant regeneration in tissue culture on glufosinate-containing medium to select transformed plants. The plasmid DNA, PHP6710, used in the transformation contained three genes: (1) DNA adenine methylase encoding dam gene; (2) the pat gene from S. viridochromogenes; and the beta-lactamase encoding bla gene, which conferred resistance to the antibiotic ampicillin and was used to select bacterial colonies transformed with plasmid DNA.

Tissue-specific expression of the dam gene was accomplished by including sequences from the maize anther-specific 512del promoter, and constitutive expression of the pat gene was regulated using the cauliflower mosaic virus (CaMV) 35S promoter.

Prior to biolistic transformation, PHP6710 plasmid DNA was subjected to restriction enzyme digestion to isolate a 4.5 kb DNA fragment containing the dam and pat genes, but not the bla gene. It was this DNA fragment, not the entire plasmid, that was used for the transformation procedure.

Characteristics of the Modification Expand

The Introduced DNA

Southern blot analyses of genomic DNA from each of the male-sterile lines confirmed that all of the lines contained one complete copy of dam and pat at a single insertion site. In addition, each line contained partial and/or rearranged copies of either the pat or dam genes. For example, line 676 contained a second pat gene at a separate insertion site; line 678 contained two additional copies of dam and one additional partial copy of pat; and line 680 contained three additional partial copies of dam. Similar analyses were also used to verify that, as predicted, the antibiotic resistance marker gene (bla) was not incorporated into any of the male-sterile lines.

Genetic Stability of the Introduced Trait

Genetic analysis of maize lines 676, 678, 680 demonstrated that the pat and dam genes segregated as a single genetic locus according to Mendelian rules of inheritance.

Expressed Material

The production of dam-specific mRNA in each of these lines was tested by Northern blot analysis, and detectable amounts were found only in anther tissue (tetrad release stage) obtained from line 678. The inability to detect dam mRNA in similar tissue from the other two lines was attributed to the fact that the expressed DAM enzyme results in cell ablation and destruction.

The levels of PAT protein expression were determined in grain and leaf tissue from each line and found to range between 204-278 µg/g total protein and 601-717 µg/g total protein in leaf tissue from lines 676 and 678, respectively. Concentrations of PAT were below the limit of detection (20 µg/g total protein) in both grain and leaf tissue from line 680. Grain from lines 676 and 678 contained low levels of PAT protein, ranging from undetectable to 19.2 µg/g total protein and 14.8 µg/g total protein, respectively.

Environmental Safety Considerations Expand

Field Testing

Outcrossing

Multiple barriers, including sterility of the maize lines 676, 678, and 680, ensured that gene introgression from these transformed lines into wild or cultivated sexually-compatible plants was extremely unlikely and such rare events would not increase the weediness potential of any resulting progeny or adversely impact biodiversity.

In the United States, where there are few plant species closely-related to maize in the wild, the risk of gene flow to other species is remote. Cultivated maize, Zea mays L. subsp. mays, is sexually compatible with other members of the genus Zea, and to a much lesser degree with members of the genus Tripsacum.

Weediness Potential

It was determined that the relevant introduced trait, male sterility, was unlikely to increase weediness of maize lines 676, 678, and 680. There was no indication that the presence of a specific DNA adenine methylase enzyme, would convert maize into a weed. As well, resistance to glufosinate ammonium would not, in itself, render maize weedy or invasive of natural habitats.

Cultivated maize is unlikely to establish in non-cropped habitats and there have been no reports of maize surviving as a weed. In agriculture, maize volunteers are not uncommon but are easily controlled by mechanical means or by using herbicides, other than glufosinate ammonium. Zea mays is not invasive and is a weak competitor with very limited seed dispersal.

Secondary and Non-Target Adverse Effects

It was determined that genetically modified maize lines 676, 678, and 680 did not have a significant adverse impact on organisms beneficial to plants or agriculture, nontarget organisms, and were not expected to impact on threatened or endangered species. Analysis of maize lines 676, 678, and 680 identified no toxic components present in concentrations significantly different from concentrations in nontransgenic maize. Furthermore, the genetic modification in maize lines 676, 678, and 680 did not result in the production of new proteins, enzymes, or metabolites, in the plant, known to have toxic properties, and plants did not exhibit any pathogenic properties.

Impact on Biodiversity

Maize lines 676, 678, and 680 have no novel phenotypic characteristics that would extend their use beyond the current geographic range of maize production. Since the risk of outcrossing with wild relatives in the United States is remote, it was determined that the risk of transferring genetic traits from these maize lines to species in unmanaged environments was insignificant. It was determined that the relative impact on plant biodiversity was neutral, as was the impact on animal and microbe biodiversity since the introduced genes were not expected to alter the plant's metabolism and as such, novel compounds would not be produced.

Food and/or Feed Safety Considerations Expand

Nutritional Data

Samples of grain and forage from transgenic and non-transgenic control lines were subjected to compositional analyses measuring the levels of protein, fat, ash, calcium and phosphorous. Forage samples were also analyzed for crude fibre content. For forage, the levels of analyzed components were found to be comparable between the male-sterile lines, between these lines and non-transgenic control lines, and to previously published literature values. With the exception of calcium levels, which were comparable between the male-sterile and non-transgenic control lines but lower than literature values, similar results were reported for the compositional analyses of grain samples. The lower calcium levels were attributed to an alteration in the analytical method.

Analyses of fatty acids (C16:0, palmitic; C18:0, stearic; C18:1, oleic; C18:2, linoleic; and C18:3, linolenic) and amino acids in samples from each male-sterile line and non-transgenic control lines did not reveal any significant differences in the respective value of each analyte. Except for the levels of a few amino acids, which were lower than those reported in the literature for commercial maize hybrids, the values measured for each analyte were within the range reported in the scientific literature. The differences noted in the levels of a few amino acids were likely due to the genetic background of the inbred parental lines relative to those lines analyzed in previously published studies.

Toxicity and Allergenicity

Previous studies have demonstrated that the PAT protein has a very low potential to be toxic or allergenic. The enzyme possesses none of the physiochemical characteristics commonly associated with known toxins or allergens, such as resistance to thermal or proteolytic degradation. The amino acid sequence of the PAT protein does not share homology with the sequences of known protein toxins and allergens, and in acute toxicity testing with laboratory mammals, adverse effects have never been observed.

Abstract Collapse

Maize (Zea mays L.), or corn, is is grown primarily for its kernel, which is largely refined into products used in a wide range of food, medical, and industrial goods.

Only a small amount of whole maize kernel is consumed by humans. Maize oil is extracted from the germ of the maize kernel and maize is also a raw material in the manufacture of starch. A complex refining process converts the majority of this starch into sweeteners, syrups and fermentation products, including ethanol. Refined maize products, sweeteners, starch, and oil are abundant in processed foods such as breakfast cereals, dairy goods, and chewing gum.
In the United States and Canada maize is typically used as animal feed, with roughly 70% of the crop fed to livestock although an increasing amount is being used for the production ot ethanol. The entire maize plant, the kernels, and several refined products such as glutens and steep liquor, are used in animal feeds. Silage made from the whole maize plant makes up 10-12% of the annual corn acreage, and is a major ruminant feedstuff. Livestock that feed on maize include cattle, pigs, poultry, sheep, goats, fish and companion animals.

Industrial uses for maize products include recycled paper, paints, cosmetics, pharmaceuticals and car parts.

The maize lines 676, 678, and 680 were genetically engineered to express male sterility and tolerance to glufosinate ammonium, the active ingredient in phosphinothricin herbicides (Basta®, Rely®, Finale®, and Liberty®). Glufosinate chemically resembles the amino acid glutamate and acts to inhibit an enzyme, called glutamine synthetase, which is involved in the synthesis of glutamine. Essentially, glufosinate acts enough like glutamate, the molecule used by glutamine synthetase to make glutamine, that it blocks the enzyme's usual activity. Glutamine synthetase is also involved in ammonia detoxification. The action of glufosinate results in reduced glutamine levels and a corresponding increase in concentrations of ammonia in plant tissues, leading to cell membrane disruption and cessation of photosynthesis resulting in plant withering and death.

Glufosinate tolerance in these maize lines is the result of introducing a gene encoding the enzyme phosphinothricin-N-acetyltransferase (PAT) isolated from the common aerobic soil actinomycete, Streptomyces viridochromogenes, the same organism from which glufosinate was originally isolated. The PAT enzyme catalyzes the acetylation of phosphinothricin, detoxifying it into an inactive compound. The PAT enzyme is not known to have any toxic properties.
The male-sterile trait was introduced by inserting a bacterial gene encoding the enzyme DNA adenine methylase (DAM). Expression of the Escherichia coli dam gene in specific plant tissues results in the inability of the transformed plants to produce anthers or pollen, resulting in a male-sterile plant. The PAT enzyme was used as a selectable marker enabling identification of transformed plants during tissue culture regeneration, and as a field selection method to identify the male-sterile lines prior to flowering. Under field conditions, plants that were not male-sterile could be eliminated by application of the herbicide glufosinate ammonium. The novel hybrid system provided an efficient and effective way to identify male-sterile plants for use in hybrid seed production.

The maize lines 676, 678, and 680 have been field tested in the major maize growing regions of the United States since 1995. Field data regarding seed germination rates, yield characteristics, disease and pest susceptibilities, compositional analyses, and numerous other reports supported the conclusion that maize lines 676, 678, and 680 were as safe to grow as any other male sterile maize and would not present a plant pest risk. It was demonstrated that the transformed maize lines did not exhibit weedy characteristics, or negatively affect beneficial or non-target organisms. Maize lines 676, 678, and 680 were not expected to impact on threatened or endangered species.

Maize does not have any closely related species growing in the wild in the continental United States and Canada. Cultivated maize can naturally cross with annual teosinte (Zea mays ssp. mexicana) when grown in close proximity, however, these wild maize relatives are native to Central America and are not naturalized in North America. Additionally, multiple barriers, including sterility of the maize lines 676, 678, and 680, ensured that gene flow from these transformed lines into wild or cultivated sexually-compatible plants was extremely unlikely. Gene exchange between maize lines 676, 678, and 680 and maize relatives was determined to be negligible in managed ecosystems, with no potential for transfer to wild species in Canada and the United States.

In an assessment of food and livestock feed safety, forage and grain from these male-sterile lines were analyzed for nutritional composition and compared to the nutritional composition of non-transgenic inbred parental lines. Proximate, fatty acid, and amino acid analyses were performed and in each case there were no significant differences noted between the transgenic and non-transgenic control lines. The use of maize products derived from lines 676, 678, and 680 was not anticipated to have any significant impact on the nutritional quality of the food supply.

Previous studies have demonstrated that the PAT protein has a very low potential to be toxic or allergenic. The enzyme possesses none of the physiochemical characteristics commonly associated with known toxins or allergens, such as resistance to thermal or proteolytic degradation, and its amino acid sequence does not show homology with the sequences of known protein toxins and allergens.

Links to Further Information Expand

U.S. Department of Agriculture, Animal and Plant Health Inspection Service

Petition for Determination of Nonregulated Status for Male-sterile Corn Lines 676, 678 and 680, with Resistance to Glufosinate Ammonium (CBI-deleted)
[PDF Size: 2.56M bytes]

US Food and Drug Administration

Memorandum to file concerning male sterile, herbicide tolerant maize.
[PDF Size: 535.70K bytes]

USDA-APHIS Environmental Assessment

Response to Pioneer Hi-Bred International, Inc.'s Petition for a Determination ofNonregulated Status for Male Sterile and Glufosinate-Tolerant Corn Lines 676, 678, and 680
[PDF Size: 57.69K bytes]

This record was last modified on Wednesday, February 25, 2015

Product Related Info

Biology Documents

Maize biology document
Biology of Maize L.
General Description

Maize is a tall, monoecious, annual grass varying in height from 1 to 4 meters (Watson & Dallwitz 1992). The main stem is made up of clearly defined nodes and internodes. Internodes are wide at the base and gradually taper to the terminal inflorescence at the top of the plant. Leaf blades are found in an alternating pattern along the stem. Maize is a unique grass as both male and female flowers are borne on the same plant but are located separately. The tassels or staminate (male) inflorescence form large spreading terminal panicles that resemble spike-like racemes. Pollen is shed from the tassel and is viable for approximately 10 to 30 minutes as it is rapidly desiccated in the air (Kiesselbach 1980). Maize plants shed pollen for up to 14 days. The reproductive phase begins when one or two auxiliary buds, present in the leaf axils, develop and form the pistillate inflorescence or female flower (Purseglove 1972). The auxiliary bud starts the transformation to form a long ‘cob’ on which the flowers will be borne. From each flower a style begins to elongate towards the tip of the cob in preparation for fertilization. These styles form long threads, known as silks. The base of the silk is unique, as it elongates continuously until fertilization occurs (Purseglove 1972). Styles may reach a length of 30 cm, the longest known in the plant kingdom. Individual maize kernels, or fruit, are unique in that mature seed is not covered by floral bracts (glumes, lemmas, and paleas) as in most other grasses, but rather the entire structure is enclosed and protected by large modified leaf bracts, collectively referred to as the ear (Hitchcock and Chase 1951). The mature female flowers will remain ready for fertilization for up to two weeks, at which point if fertilization has not occurred, the nucleus will de-organize and fertilization will no longer be possible (Hitchcock & Chase 1951). The pollen of maize, a protandrous plant, matures before the female flower is receptive (Purseglove 1972). This may have been an ancient mechanism to ensure cross-pollination, but is no longer considered conducive to modern agricultural practices. However, decades of conventional selection and improvement have produced many maize varieties with similar maturities for both male and female flowers, to ensure seed set for agricultural proposes.

Reproduction

Under natural conditions, maize reproduces only by seed production. Pollination occurs with the transfer of pollen from the tassels to the silks of the ear. About 95% of the ovules are cross-pollinated and about 5% are self-pollinated (Poehlman 1959), although plants are completely self-compatible. Maize, as a thoroughly domesticated plant, has lost all ability to disseminate its seeds and relies entirely on the aid of man for its distribution (Stoskopf 1985). The kernels are tightly held on the cobs and if ears fall to the ground, so many competing seedlings emerge that the likelihood that any will grow to maturity is extremely low. Maize cannot reproduce asexually by natural means. It is possible to reproduce maize using tissue culture techniques, however, it has proven extremely difficult with a low rate of success (Hoisington et al. 1998).

Classification and Phylogeny of Maize

Zea is a genus belonging to the grass family, Poaceae, in the Andropogoneae tribe, but is commonly cited in literature to belong to the tribe Maydeae. Zea (zeia) was derived from an old Greek name for a food grass. The genus Zea consists of four species of which only Zea mays ssp. mays L. is economically important. The other Zea species, referred to as teosintes, are largely wild grasses native to Mexico and Central America (Doebley 1990). The number of chromosomes in Zea mays is 2n=20, 21, 22, 24 (FAO 2000c). The tribe Maydeae consists of seven genera: two of American origin, Zea and Tripsacum; and five of Eastern origin which extend from India through Southeastern Asia to Australia and include Coix, Sclerachne, Polytoca, Chionachne and Trilobachne (Watson & Dallwitz 1992). It is generally agreed that maize phylogeny was largely determined by the American genera Zea and Tripsacum, however it is accepted that the genus Coix contributed to the phylogenetic development of the species Z. mays (Radu et al. 1997). The genus Manisuris, from the tribe Andropogoneae, may also have contributed to the evolution of Zea mays (Eubanks 1997a; Radu et al. 1997).

Center of Origin and Progenitors of Maize

The center of origin for Zea mays ssp. mays has been established as the Mesoamerican region, now Mexico and Central America (Watson & Dallwitz 1992). The exact period of domestication and the ancestors from which maize arose are unclear. Archaeological records suggest that domestication of maize began at least 6000 years ago, occurring independently in regions of the southwestern United States, Mexico, and Central America (Mangelsdorf 1974). The origins of domesticated maize have been difficult to trace as hybridization events in its evolution are thought to have involved a now extinct wild maize ancestor of which little evidence has been found (Eubanks 1997a).

Germplasm Diversity

A study in 1992 concluded that maize landraces occupied 42% of the land under production in less developed countries (México 1994). Mexico and Central America are the only regions where ancient maize lines, such as pod corn, are found (Mink & Dorosh 1985). The survival of maize landraces has been attributed to the integral role maize sustains in small communities where it has very specific uses for food and other special functions. These landraces have been well characterized in Latin America, and germplasm not yet evaluated is known to be present in the region. There is a growing trend in developing countries to adopt improved maize varieties, primarily to meet market demand. In Mexico, only 20% of the corn varieties grown 50 years ago remain in cultivation (World Watch Institute 2000). The narrowing of genetic diversity in modern maize varieties emphasizes the importance of conserving genetic traits for future plant breeding. Leading the way in preserving maize germplasm is CIMMYT (International Maize and Wheat Improvement Centre), which has the world’s largest collection of maize accessions, with over 17,000 lines (CIMMYT 2000). Collections of distant maize relatives native to Asia have been recommended by conservation experts to ensure their preservation for future research (Sharma 1998).

Relatives of Maize

The closest wild relatives of maize are the teosintes which all belong to the genus Zea. The teosintes are wild grasses native to Mexico and Central America and have limited distribution (Mangelsdorf et al. 1981). Teosinte species show little tendency to spread beyond their natural range and distribution is restricted to North, Central and South America (Watson & Dallwitz 1992). The nearest teosinte relative to Z. mays ssp. mays is Z. mays ssp. mexicana (Schrader) Iltis (previously classified as Euchlaena mexicana, Zea mexicana) (2n = 20). This Central Mexican annual teosinte is a large flowered, mostly weedy grass with a broad distribution across the central highlands of Mexico. It does not spread readily. It has limited use as a forage and green fodder crop, but can be problematic due to weedy tendencies (Doebley 1990, Watson & Dallwitz 1992). The closest wild relatives to maize outside the Zea genus are from the genus Tripsacum. The genus Tripsacum is comprised of about 12 species that are mostly native to Mexico and Guatemala but are widely distributed throughout warm regions in the USA and South America, with some species present in Asia and Southeast Asia (Watson & Dallwitz 1992). All species are perennial, warm season grasses.

Outcrossing

Gene flow from maize can occur by two means: pollen transfer and seed dispersal. Seed dispersal can be readily controlled in maize as domestication has all but eliminated any seed dispersal mechanisms that ancestral maize may have previously used (Purseglove 1972). The kernels are held tightly on the cobs and if the ear falls to the ground, competing seedlings limit growth to maturity (Gould 1968). Pollen movement is the only effective means of gene escape from maize plants. Pollen is released from the tassels at the top of the plant and transported by wind to the female flowers located on the stalk. Pollen shed occurs over a 14 day period, with a peak during the first 5 days of shed (Sears 2000). The stigmas are receptive for a large part of this two week interval (Kiesselbach 1980). Insects, such as bees, have been observed to collect pollen from maize tassels, but they do not play a significant role in cross-pollination as there is no incentive to visit the female flowers (Rayor et al. 1972).

Wind speed and direction affect pollen distribution

Differences in flowering dates among commercial maize varieties are small. Cross-pollination between varieties may occur if grown in adjacent fields. The limited viability of maize pollen reduces the risk of cross-pollination, as a receptive host must be found within the 30 minutes that the pollen remains biologically active. Cross-pollination is also affected by the concentration of maize pollen released; pollen produced by a maize crop will successfully compete with foreign pollen sources when present in higher concentrations (Rayor et al. 1972). The isolation of crops using separation distances and physical barriers are common techniques for restricting gene flow and ensuring seed purity for maize seed production. Cross-pollination is controlled in seed lots by separating different lines. The OECD standards require a minimum 200 m (660 ft) isolation distance for maize seed production. This distance has been estimated to maintain inbred lines at 99.9% purity. Physical barriers, such as trees or barrier rows, are also effective in reducing cross-pollination. Detasseling or bagging the tassel effectively prevents pollen movement and limits gene flow.

Intraspecific hybidization

All forms of Zea mays spp mays, such as dent, sweet, and pop corn, freely cross pollinate forming fertile hybrids (Pursglove 1972).

Interspecific hybridization

All teosintes can be crossed to maize and form fertile hybrids, except for the tetraploid Z. perennis. Maize-teosinte hybrids exhibit low fitness and have little impact on gene introgression in subsequent generations (Galinat 1988). The tendency to form natural hybrids differs among teosintes: Z. luxurians rarely hybridizes with maize, whereas Z. mays ssp. mexicana frequently forms hybrids (Wilkes 1977). Molecular data confirms that there is gene flow between maize and teosinte and suggests that introgression of maize and teosinte occurs in both directions but at low levels (Doebley 1990).

Intergeneric hybridization Tripsacum species (T. dactyloides, T. floridanum, T. lanceolatum, and T. pilosum) have been successfully hand crossed with maize to form hybrids. The hybrids generally have 28 chromosomes, 10 from maize and 18 from Tripsacum, and are pollen sterile with limited female fertility (Mangelsdorf & Reeves 1939, Mangelsdorf 1974). This infertility is common in such wide crosses because of differences in chromosome number and lack of pairing between chromosomes (Eubanks 1997b). Maize readily crosses with hexaploid wheat (Triticum aestivum) with high frequencies of fertilization and embryo formation (plant breeders use maize pollen to develop double haploids of hexaploid wheat). However, maize chromosomes are eliminated from the genome during the initial stages of meiosis and result in haploid embryos. There is little evidence to suggest fertile hybrids between maize and hexaploid wheat could be produced in nature. There have been unsubstantiated reports of hybridization between maize and sugar cane (Saccaharum sp.).

Biology References
1. CIMMYT (International Maize and Wheat Improvement Centre) (2000). CGIAR Research, Areas of Research: Maize (Zea mays L.) http://www.cgiar.org/areas/maize.htm
2. Doebley, J. F. (1990). Molecular Evidence for Gene Flow among Zea species. BioScience 40, 443- 448.
3. Eubanks, M.W. (1997a). Further evidence for two progenitors in the origin of maize. Maize Newsletter 71, 35-35.
4. FAO (2000c). Species Description. Zea mays L. http://www.fao.org/WAICENT/faoinfo/agricult/agp/agpc/doc/gbase/data/pf000342.htm
5. Galinat, W.C. (1988). The origin of corn. In: Corn and Corn Improvement. Agronomy Monographs No.18. American Society of Agronomy, G.F Sprague and J.W. Dudley, (eds.). Madison, Wisconsin, pp. 1-31.
6. Gould, F.W. (1968). Grass Systematics. McGraw Hill, N.Y., USA.
7. Hitchcock, A.S. & A. Chase. (1951). Manual of the Grasses of the United States (Volume 2). Dover Publications: N.Y. p. 790-796.
8. Hoisington, D., Listman, G.M. & Morris, M.L. (1998). Varietal development: applied biotechnology. In: Maize Seed Industries in Developing Countries. M. L. Morris (ed). Lynne Rienner Publishers, Inc. pp. 77-102.
9. Kiesselbach, T.A. (1980). The structure and reproduction of corn. Reprint of: Research Bulletin No. 161. 1949. Agricultural Experiment Station, Lincoln, Nebraska. University of Nebraska Press. p. 93.
10. Laurie, D.A. & Bennett, M.D. (1986). Wheat x maize hybridization. Can. J. Genet. Cytol. 28, 313-316.
11. Mangelsdorf, P. C. & Reeves, R.G. (1939). The origin of Indian corn and its relatives. Texas Agric. Expt. Sta. Bull. 574.
12. Mangelsdorf, P. C. (1974). Corn: its Origin, Evolution and Improvement. Harvard Univ. Press, Cambridge, Mass.
13. Mangelsdorf, P. C., Roberts, L.M. & Rogers, J.S. (1981). The probable origin of annual teosintes. Bussey Inst., Harvard Univ. Publ. 10, 1- 69.
14. México, D.F. (1994). Maize seed industries revisited: emerging roles of the public and private sectors. World Maize Facts and Trends1993/94. CIMMYT.
15. Mink, S. D. & Dorosh, P.A. (1987). An overview of corn production. In: The Corn Economy of Indonesia. Cornell University Press, London.
16. Poehlman, J.M. (1959). Breeding Field Crops. Holt, New York, USA.
17. Purseglove, J.W. (1972). Tropical Crops: Monocotyledons 1. Longman Group Limited., London.
18. Radu, A, Urechean, V., Naidin, C. & Motorga, V. (1997). The theoretical significance of a mutant in the phylogeny of the species Zea mays L. Maize Newsletter 71, 77-78.
19. Rayor, G.S., Ogden, E.C. & Hayes, J.V. (1972). Dispersion and deposition of corn pollen from experimental sources. Agronomy Journal 64, 420-427.
20. Sears, M.K., Stanley-Horn, D.E. & Matilla, H.R. (2000). Ecological impact of Bt corn pollen on Monarch butterfly in Ontario. Canadian Food Inspection Agency (http://www.cfia-acia.agr.ca/english/ plaveg/pbo/btmone.shtml)
21. Sharma, A.B. (1998). Experts for conservation of wild crop varieties. The Financial Express. Indian Express Group.
22. Stoskopf, N.C. (1985). Cereal Grain Crops. Reston, Virginia: Reston Publishing Company, Inc., Prentice-Hall Company.
23. Watson, L. & Dallwitz, M.J. (1992). Grass Genera of the World: Descriptions, Illustrations, Identification, and Information Retrieval; including Synonyms, Morphology, Anatomy, Physiology, Phytochemistry, Cytology, Classification, Pathogens, World and Local Distribution, and References. Version:18th August 1999. http://biodiversity.uno.edu/delta
24. World Watch Institute (2000). Crop gene diversity declining. Trends in Plant Science 5, 7.
25. Wilkes, H.G. (1977). Hybridization of maize and teosinte, in Mexico and Guatemala and the improvement of maize. Economic Botany 31, 254-293.
LAST UPDATED: SEPTEMBER 2002

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Summary of Introduced Genetic Elements Expand

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Characteristics of the Modification Expand

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